Staphylococcus aureus is a versatile pathogen that has successfully adapted and evolved to produce a variety of virulence factors that allow it to colonize and subsequently cause a variety of diseases in humans. Methicillin-resistant S. aureus (MRSA), once a scourge of modern hospitals primarily, has found a niche in community settings in people without any known risk factors. Our long-term research goal is to understand the genetic factors that are influencing the changing epidemiology of community-associated MRSA (CA- MRSA). Our specific hypothesis for this proposal is that the 1998 hypervirulent 'prototype' CA-MRSA strain, MW2 (caused fatal septicemia in a 16-month old American Indian girl in 1998 in North Dakota, USA), evolved from a specific Wisconsin CA-MRSA major clonal group-2 (MCG-2), which was first isolated in 1992 in a Native American community (NAC) in Wisconsin. We further hypothesize that the MCG-2 strains have evolved to acquire specific genetic traits from methicillin-sensitive S. aureus (MSSA) strain in NACs that allowed them to become more adaptable, successful, and virulent pathogens. The specific aims are to: 1) Determine if the early (1992) CA-MRSA strains belonging to the MCG-2 are the progenitors of the successful CA-MRSA strain, MW2. Specifically, we will determine if the two Native American CA- MRSA strains, WI-34 and WI-99, from Wisconsin were the progenitor strains of MW2. We will compare the genomes of WI-34 and WI-99, with that of the 'prototype' CA-MRSA, MW2 strain using a custom-made MW2 GeneChip[unreadable] from Affymetrix Inc. Additionally, we will thoroughly genotype WI-34 and WI-99 by several genotypic methods. 2) Determine whether or not contemporary CA-MRSA strains belonging to clonal group, MCG-2 evolved from MSSA strains circulating in five NACs in Wisconsin. We will genotype: i) 75 MRSA from MCG-2 (retrospective collection), ii) 75 NAC-MSSA strains (retrospective collection), iii) 75 MSSA strains from a non-NAC (retrospective collection), iv) 75 most recently collected NAC-CA-MRSA (years 2002-04) and v) 20 transitional MRSA strains (retrospective collection) that have some but not all features of CA- MRSA by pulsed-field gel electrophoresis (PFGE), multilocus sequence type (MLST), staphylococcal cassette chromosome mec (SCCmec), spa and coa typing. 3) Determine whether or not the contemporary CA-MRSA have evolved or continue to evolve by acquiring additional virulence factors from other MRSA/MSSA to become awidely successful pathogen. We will determine the presence or absence of 40 different virulence factor genes in all 320 isolates listed in aim 2 and determine evolution of virulence factor genes in contemporary CA-MRSA.